I have managed to get all /admin requests redirected to https by:

server {
    listen 80;

    location /admin {
        rewrite ^ https://$server_name$request_uri?$args permanent;
    }
}

But can't figure out how to get all https requests that are not within /admin redirected to http, so far I have:

server {
    listen       443;

    location ~ /admin {
        rewrite ^ http://$server_name$request_uri?$args permanent;
    }
}

EDIT:

I have got the redirects working as required but can't stop the /admin url going to 404. It feels like I need to put something in the empty block.

server {
    listen       443;

    location /admin {

    }   

    location / {
        rewrite ^ http://$server_name$request_uri?$args permanent;
    }
}

Thanks